1. Field of the Invention
The invention relates to a method for manufacturing foams, especially microcellular foams, from polymers or polymer mixtures for shaping molded bodies whereby the polymers or polymer mixtures are melted on the assembly line before they are passed through a shaping device in an extruder device and foam formation takes place by introducing a gas, as well as molded bodies manufacturable by means of the method.
2. Discussion of Related Art
Such a method by which the widest variety of molded bodies can be made by melting extrusion with the aid of a foam-forming gas have been known for a long time for a very wide variety of applications. The previous methods however are regularly characterized by the fact that the energy expenditure to work the method was and is very great. For example, a method is known from DE-OS 44 37 800 in which the pressure load in the extrusion device therein is constant over the temperature that is different in two zones so that an impregnation of the melt with a foam-forming gas is not possible.
From DE-OS 44 37 860, a method is known for manufacturing sheets of web-microcellular foams from amorphous thermoplastic plastic by impregnation of a plastic melt with a volatile propellant in which, in a first extrusion zone, the thermoplast is impregnated at a temperature above its glass temperature with a propellant and in which, in a second extrusion zone, the propellant-containing melt is cooled to a temperature that is above the glass temperature of the propellant-containing thermoplast. The propellant-containing melt is thereby cooled by at least 40xc2x0 C. to a temperature that is at least 30xc2x0 C. above the glass temperature of the propellant-containing thermoplast.
In other methods for manufacturing foams from polymers, so-called nucleating agents, in the form of talcum or zeoliths for example, or an additional gas, are necessary.
Overall, the previous methods are not capable either because of their regularly high energy consumption and/or possible additional means for initiating the formation of foam and are not able to produce satisfactory results relative to the products made by the method, so that it is the goal of the present invention to provide a method of the type described at the outset with which molded bodies can be made continuously with an energy requirement that is very low and free of additional substances or materials, i.e., in a continuous process whereby the method is intended to be able to produce both open-celled and closed molded bodies and with which an adjustment of the pore size is possible with considerable and desired accuracy and whereby the method is simple and can be performed economically by comparison with the known methods.
This goal is achieved according to the invention by the fact that the process line is divided into at least a first part and a second part whereby in the first part, the melt of the polymer or polymer mixture and the addition of the gas with pressure greater than the melt takes place under the influence of a shearing and/or kneading and/or homogenization means on the polymer or polymer mixture charged with gas and whereby, at the end of the second part of the process line following the first part, foam formation of the gas-charged polymer or polymer mixture having a higher pressure than the pressure in the first part takes place.
The advantage of the method according to the invention consists essentially in the fact that the segmentation chosen according to the invention in lower pressure and higher temperature-influenced zones and in higher pressure and lower temperature-influenced zones makes possible the high solubility of the gas. This therefore permits optimum foam formation whereby the amount of gas in the melt which can be adjusted in volume according to the desired property of molded bodies manufactured according to the method, makes possible by the resultant reduction in the viscosity of the melt, a reduction of the torque to be used, and therefore a reduction of the energy consumption for working the method or a device with which the method can be performed.
The advantage of the method according to the invention also lies in the fact that, by dividing or segmenting the process line, the process of foam formation can be influenced under control, while, before the pressure of the melt is increased, the polymer or polymer mixture is melted and the gas can be introduced and after the pressure is increased in the area between the pressure-increasing means and the shaping device, foam formation, especially microcellular foam formation, can take place under control.
In an advantageous embodiment of the method, the temperature of the gas introduced into the first part of the process line is greater than the glass or melting temperature of the polymer or of the polymer mixture so that the gas introduced initially can distribute itself very well in the polymer or polymer mixture melt. The temperature in this process line is then advantageously chosen so that immediately after adding the gas, in other words the actual mixing process of the gas into the polymer or the polymer mixture, further treatment of the gas-charged polymer or polymer mixture melt advantageously takes place with respect to the above-mentioned shearing, kneading, and homogenization process while retaining the melting temperature of the polymer or polymer mixture.
The pressure of the gas introduced into the polymer or polymer mixture melt preferably is greater than 150 bars, whereby the solubility of the gas introduced into the polymer or polymer mixture melt is increased.
Preferably, the temperature of the gas-enriched polymer or polymer mixture between the second part of the process line and the shaping device is reduced relative to the temperature of the gas-enriched polymer or polymer mixture in the first part of the process line, whereby it is also advantageous to increase the pressure of the gas-enriched polymer or polymer mixture between the second part and the shaping device relative to the pressure of the gas-enriched polymer or polymer mixture in the first part of the process line. The pressure increase at the end of the first part of the process line by pressure-increasing means, for example in the form of a gear melt pump, makes it possible to reduce the temperature of the gas-charged polymer or polymer mixture melt so that a change in viscosity can be effected in a wide range.
Advantageously, the pressure of the gas-enriched polymer or polymer mixture between the second part of the process line and the shaping device is in the range of up to 1500 bars, and preferably at least 500 bars. Experiments have shown that such pressures are sufficient for a completely homogeneous dissolution of the gas in the polymer or polymer mixture melt, whereby preferably the temperature in this area is up to 150xc2x0 C. below the temperature in the first part of the process line.
The adjustment of pressure in the first and/or second part of the process line is preferably adjusted as a function of the type of polymer or polymer mixture and/or a desired pore structure (pore size, open pores, closed pores), whereby all amorphous thermoplastic and partially crystalline polymers, copolymers, and polymer blends such as polycarbonate, polysulfone, polyethersulfone, polypropylene, polyethylene, polyamide, polyester, PVDF, etc., can be used as the polymers and whereby the polymer mixtures can be mixtures of the polymers mentioned above, for example.
In addition to adjusting the pressure, it is likewise advantageous to adjust the temperature of the polymer or the polymer mixture, i.e., its melts and/or gas-enriched melts, in the first and/or second part of the process line, possibly as a function of the type of polymer or of the polymer mixture and/or a desired pore structure, in order to obtain an optimum result for the respective polymer and/or polymer mixture and/or the resultant product with respect to the type of pores (closed, open) and the size of the pores, which simultaneously can serve to reduce energy consumption.